syouziki ni iu to, kono heya ha sukosi semai desu.

Yes, it is natural.

Japanese is normally written without spaces, so the usual form is:

正直に言うと、この部屋は少し狭いです。

A few notes:

• せまい is often written as 狭い
• the comma after と is normal
• the meaning stays the same whether せまい is in kana or kanji

Here, 正直 is being used adverbially, so it takes に.

• 正直 = honesty / honest
• 正直に = honestly
• 正直に言う = to say honestly

This is a very common pattern with na-adjectives and similar words:

• しずか → しずかに
• ていねい → ていねいに
• 正直 → 正直に

So 正直に modifies 言う.

In this expression, と is part of a set phrase meaning something like to speak honestly or if I say it honestly.

So:

• 正直に言うと = to be honest / honestly speaking

It is not translating word-for-word into natural English. Japanese often uses this kind of structure where the literal grammar is a little different from the best English translation.

A similar expression is:

• 簡単に言うと = simply put
• 別の言い方をすると = to put it another way

Because この must be placed directly before a noun.

• この部屋 = this room
• これ = this one

So:

• この
  • noun
• これ by itself

Examples:

• この本 = this book
• これ = this one

That is why この部屋 is correct.

は marks この部屋 as the topic of the sentence.

The sentence is basically saying:

• As for this room, it is a little small.

Using は is very natural when you are giving a comment or description about something already known in the conversation.

If you used が instead, the nuance could change. が often sounds more like:

• identifying something
• emphasizing the subject
• introducing new information

So in this sentence, この部屋は is the most normal choice.

少し means a little or slightly.

Here it softens the statement:

• この部屋はせまいです = This room is small/cramped.
• この部屋は少しせまいです = This room is a little small/cramped.

It makes the sentence sound less direct.

In conversation, ちょっと is also very common:

• この部屋はちょっと狭いです。

That sounds a bit more casual and conversational than 少し.

Yes. です can follow an い-adjective in polite speech.

• せまい = plain/casual
• せまいです = polite

This is normal Japanese.

Important:

• Do not add だ after an い-adjective
• so せまいだ is wrong

Examples:

• 高いです = it is expensive/high
• 暑いです = it is hot
• 狭いです = it is narrow/small

Yes. It is usually written 狭い.

So:

• せまい = kana writing
• 狭い = kanji writing

Both are read the same way and mean the same thing. Beginner materials often use more kana to make reading easier.

Japanese often leaves out the subject when it is understood from context.

In 正直に言うと, the speaker is the one doing the saying, so I is understood automatically.

Japanese does this all the time. For example:

• 行きます。 = I go / I’m going / I will go
• わかりません。 = I don’t understand

So even though English often needs I, Japanese often does not say it.

No, there are several similar expressions.

Common alternatives:

• 正直言うと — more casual, often heard in speech
• 正直に言えば — if I speak honestly / to be honest
• ぶっちゃけ — very casual, slangy frankly

For the second half, you might also hear:

• この部屋はちょっと狭いです。

That often sounds very natural in everyday conversation.

So your original sentence is good, but there are other natural variations depending on tone and formality.